The present invention relates to an objective lens which can be used in a optical disc system such as writing and/or reading device and device for creating a master of an optical disc.
Conventionally, optical disc reading/writing devices have been widely used. An optical disc reading/writing device is typically configured such that a laser beam emitted by a laser source is converged at a point in the vicinity of data recording surface of the optical disc to readout/write the information thereon. In order to perform the reading/writing operation efficiently, it is preferable that the quantity of light converged on the data recording surface is as much as possible. For this purpose, an objective lens for the optical disc reading/writing device is generally formed with a reflection suppressing coating on its surface. An example of such a coating is disclosed in Japanese Patent Provisional Publication No. P2001-52366A.
Generally, the reflection suppressing coating formed on the surface of the objective lens is configured such that its reflectivity for a ray of light at a predetermined wavelength (which will be referred to hereinafter as a design wavelength) incident thereon is minimum when an incident angle of the ray is zero degree. It is known that, for the wavelength of light different from the design wavelength, the reflection suppressing coating exhibits higher reflectivity (i.e., the reflection suppressing capability is lowered). Further, due to the inclination of the surface of the objective lens, an incident angle of a ray increase at an outer portion of the lens surface. As the incident angle of a ray is varied, the wavelength for which the reflection suppressing coating exhibits the minimum reflectivity is shifted to a shorter wavelength side. In other words, when rays having the same wavelength are incident on the central area (an area around the optical axis) and peripheral area (an area surrounding the central area) of the objective lens, the peripheral area exhibits a higher reflectively than the central area.
In the above-identified publication, in order to deal with the problem of varying the reflectivity due to the inclination of the lens surface, a lower design wavelength is employed for forming the reflection suppressing coating. With this configuration, increase of the reflectivity at the peripheral area is suppressed. Although the reflectivity at the central area increases according to the above configuration, the reflectivity as a whole (i.e., for the rays of light incident on an effective range of the objective lens) is well suppressed.
Recently, an optical disc having a higher recording density (i.e., having a larger data capacity) has been developed and used (e.g., DVD: digital versatile disc). When data is written onto and/or readout from such a disc, in accordance with its higher recording density, a high NA objective lens is used. By employing the higher NA lens, the diameter of a beam spot is well reduced.
The objective lens disclosed in the above-indicated publication is configured to reduce the reflectivity with respect to the light incident thereon so that loss of light is suppressed.
In an optical disc, since a pitch of adjacent data tracks is relatively small. In particular, an optical disc whose density is higher than that of the DVD has been suggested recently (e.g., Blue-Ray disc), which requires a higher NA lens. For such a high-density optical disc, it is desired that the intensity of a side lobe formed around the beam spot is well reduced.